Exhaust gas sensors may be operated to provide indications of various exhaust gas constituents. For example, U.S. Pat. No. 5,145,566 describes detecting water content in the exhaust gas.
The inventor herein has recognized various additional information that can be obtained from manipulation of an exhaust gas sensor, including information relating to a fuel alcohol content of a fuel burned in the engine. Thus, in one example, a method for an exhaust gas sensor coupled to an exhaust system of an engine is disclosed. The method comprises, during selected engine fueling conditions, alternating between applying first and second voltages to the sensor; and identifying an amount of alcohol in fuel injected to the engine based sensor outputs at the first and second voltages.
Thus, in one example, the sensor outputs may be used to correlate exhaust water content to the fuel alcohol content. Specifically, responsive to application of the first and second voltages, first and second pumping currents may be generated. The first pumping current may be indicative of an amount of oxygen in a sample gas while the second pumping current may be indicative of the amount of oxygen in the sample gas plus an amount of oxygen contained in water molecules in the sample gas. As such, the amount of oxygen indicated by the first pumping current may be subtracted from the amount of oxygen plus the amount of oxygen contained in water molecules to obtain an indication of the amount of water in the exhaust gas. In this way, the fuel alcohol content may be identified based on the amount of water in the exhaust gas.
Further, the inventor has recognized that various external factors can confound the fuel alcohol content measurement when using exhaust gas sensors, such as exhaust gas oxygen sensors. For example, ambient humidity changes and/or exhaust gas recirculation (EGR) can affect the exhaust water content and thus degrade the fuel alcohol content identification. As such, to reduce disturbances on such a measurement, ambient humidity information may also be used in identifying the fuel alcohol content. In one particularly advantageous approach, the exhaust gas sensor itself, or another exhaust gas sensor, may be used to determine ambient humidity, for example, when the engine is operating without fueling (e.g., deceleration fuel shut-off), or when fuel alcohol content of the fuel is otherwise known and unchanging (e.g., during a condition other than after a fuel tank re-fill). Likewise, the sensor outputs may be used to determine alcohol content when external EGR is disabled, so that effects on exhaust water content due to varying levels of EGR are reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.